Nanotubes are members of the fullerene structural family, which also includes the spherical buckyballs, and the ends of a nanotube may be capped with a hemisphere of the buckyball structure. Their name is derived from their long, hollow structure with the walls formed by one-atom-thick sheets of carbon, called graphene. These sheets are rolled at specific and discrete (“chiral”) angles, and the combination of the rolling angle and radius decides the nanotube properties; for example, whether the individual nanotube shell is a metal or semiconductor. Nanotubes are categorized as single-walled nanotubes (SWCNT) and multi-walled nanotubes (MWNT). While single-walled carbon nanotubes consist of one single folded graphene sheet, multiple walled carbon nanotubes of multiple rolled layers (concentric tubes) of graphite.
Single-walled carbon nanotubes (SWCNT) are characterized by their unique mechanical, electrical and optical properties. The tensile strength of individual SWCNT can be well over 30 GPa and electrical conductance of metallic SWCNT ropes approach 106 S/m. Formed after deposition of SWCNT dispersions, SWCNT networks also allow visible and infrared light transmission in the direction normal to the plane of the film. This property arises from the extremely small diameters (<1.5 nm average) of the SWCNT coupled with the huge aspect (i.e., length-to-diameter) ratio with typical values of 1000-1500. Thus, the formation of transparent conductive networks is possible. The combination of such properties in a single material marks them as distinctive candidates for a multitude of lab-demonstrated applications like field effect transistors, non-volatile memories, displays, touch screens, battery electrodes, supercapacitors and filtration membranes.
Emerging industrial applications for SWCNT include polymer composites with better strength performance (Coleman, J. N.; Khan, U.; Blau, W. J. and Gun'ko, Y. K., Small but strong: A review of the mechanical properties of carbon nanotube-polymer composites. Carbon 2006, 44, 1624-1652); allowing for new concepts in the aerospace industry, (Jacoby, M., Composite Materials. C & EN, Aug. 30, 2004, pp. 34-39); electronics (e.g., data storage, displays, sensors, thin film transistors) (Cao, Q. and Rogers, J. A., Ultrathin Films of Single-Walled Carbon Nanotubes for Electronics and Sensors: A Review of Fundamental and Applied Aspects. Adv. Mater. 2008, 20, 29-53) (Avouris, P., Carbon Nanotube Electronics and Photonics. Physics Today 2009, 34-40); batteries (Landi, B. J.; Ganter, M. J.; Schauerman, C. M.; Cress, C. D. and Raffaelle, R. P., Lithium Ion Capacity of Single Wall Carbon Nanotube Paper Electrodes. J. Phys. Chem. C 2008, 112, 7509-7515) (Pushparaj, V. L.; Shaijumon, M. M.; Kumar, A.; Murugesan, S.; Ci, L.; Vajtai, R.; Linhardt, R. J.; Nalamasu, O. and Ajayan, P. M., Flexible Energy Storate Devices Based on Nanocomposite Paper. Proc Natl Acad Sci USA 2007,101,13574-13577); supercapacitors; filter membranes for the removal of viral and bacterial pathogens (Brady-Estevez, A. S.; Kang, S. and Elimelech, M., A Single-Walled-Carbon-Nanotube-Filter for Removal of Viral and Bacterial Pathogens. Small 2008, 4, 481-484); detection of chemical and biological species (Heller, D. A.; Jin, H.; Martinez, B. M.; Patel, D.; Miller, B. M.; Yeung, T.-K.; Jena, P. V.; Höbartner, C.; Ha, T.; Silverman, S. K. and Strano, M. S., Multimodal Optical Sensing and Analyte Specificity using Single-Walled Carbon Nanotubes. Nature Nanotechnology 2009, 4, 114-120); including potential warfare agents (Lee, C. Y.; Sharma, R.; Radadia, A. D.; Masel, R. I. and Strano, M. S., On-Chip Micro Gas Chromatograph Enabled by a Noncovalently Functionalized Single-Walled Carbon Nanotube Sensor Array. Angew. Chem. Int. Ed. 2008, 47, 5018-5021); and transparent conducting electrodes (Eikos, Inc., 2009, www.eikos.com) (Unidym, Inc., 2009, www.unidym.com), e.g., for LCDs, touch screens, and flexible solar cells (Contreras, M. A.; Barnes, T.; van de Lagemaat, J.; Rumbles, G.; Coutts, T. J.; Weeks, C.; Glatkowski, P.; Levitsky, I.; Peltola, J. and Britz, D., Replacement of Transparent Conductive Oxides by Single-Wall Carbon Nanotubes in Cu(In, Ga)Se2-Based Solar Cells. J. Phys. Chem. C 2007, 111, 14045-14048). Despite the great potential across sectors and applications, announcements of the commercialization of sensors (Halford, B., Carbon Nanotube Electronics Power Up. C&EN, Jan. 3, 2005, p. 27) and the presentation of prototypes of field effect displays and flat panel TV screens (Kanellos, M. Carbon TVs to Edge out Liquid Crystal, Plasma? NEWS.COM, 01/05/2005), there is no evidence of significant use of SWCNT at industrial scale. However, previous predictions (Frost & Sullivan, An Assessment on the Future of Carbon Nanotubes—Strategic of the Market and Potential. June 2004) of a dramatic market growth from less than one million dollars in 2001 to $200 million in 2007 have not materialized. Enabling the broad use of SWCNT requires their availability in a form which allows their easy addition and sufficiently homogeneous mixing with hosts such as other chemicals or homogenous deposition on substrates without loosing or able to recover beneficial properties of SWCNT such as mechanical strength or electrical conductivity.
A major limitation of the chemical modification of carbon nanotubes is their very limited solubility in organic solvents, which does not allow for homogeneous solution-based reactions, which are necessary for commercial and industrial applications of nanotubes. Accordingly, there is a need for a solution that will improve the ability to utilize nanotubes without sacrificing their beneficial properties.
Carbon nanotubes possess unique properties making them useful, among others, for the enhancement of electrical, but also thermal conductivity. Of particular interest is maintaining optical transparency (e.g., the absence of absorption and also scattering) for electromagnetic waves of targeted wavelengths while achieving high electrical conductivity. Transparent conducting electrodes for photovoltaic (and other) devices are only one example for which such properties are critical. However, to take advantage of these unique properties, miscibility with or dispersibility in a range of hosts is necessary. Mechanical mixing in the solid state has been used to utilize the carbon nanotubes, but usually does not allow for sufficient mixing at the nanoscale level as van der Waals force based attraction between individual SWCNT (leading to bundles) is often stronger than attractive forces between SWCNT and the host. Surfactants such as, but not limited to, sodium cholate and sodium dodecyl sulfate (SDS), can wrap around individual SWCNT and break the bundling forces, e.g., leading to debundling, also called exfoliation. However, to achieve this result significant quantities of surfactant are required, which often negatively affect the characteristics of the resulting mixture. Liquid dispersions of carbon nanotubes have also been attempted. In some cases, liquid dispersions have been achieved by at least partially exfoliating (e.g., debundling) individual carbon nanotubes.
After their formulation, such carbon nanotube dispersions can be either mixed with solutions of other materials, e.g., polymers of which electrical conductivity is intended to be increased, or deposited on substrates using established coating techniques such as dip- and spray-coating or inkjet printing.
However, carbon nanotubes are, at best, only poorly soluble in almost all known solvents. In many cases, carbon nanotubes are not soluble at all, e.g., no visible coloration can be achieved after precipitation of the added SWCNT, consistent with a concentration of <0.01 mg/mL. As a result, solubility enhancement is necessary.
Chemical functionalization is another route to achieve enhanced solubility in selected solvents. The type of the functional group will affect the identity of the solvent that is suitability for creating a dispersion. Functionalization strategies are generally divided into two different approaches (e.g., X. Peng and S. S. Wong, Adv. Mater. 2009, 625-642). In the first approach, oxygenated chemical groups such as carboxylic acid (—COOH) are introduced onto the tips and defect sites of nanotube surfaces by treatment with nitric acid (HNO3). Amidation and esterification are used for the subsequent linking of molecular moieties such as alkyl chains, polyethylene glycol, aromatic species or bio-inspired moieties. In the second approach, covalent sidewall reactions are carried out on the nanotube surface. Polar, pericyclic as well as radical reactions are used to construct carbon-carbon and carbon-heteroatom bonds on the surface of carbon nanotubes. Examples of organic functionalization strategies include [2+1] cycloadditions of nitrenes and carbenes, 1,3-dipolar cycloadditions of azomethine ylides and reactions with diazonium compounds.
While reactions are often similar to those of C60 and C70 fullerenes, differences exist. For example, [4+2] cycloadditions such as the Diels-Alder cycloaddition of o-quinodimethane has been reported. However, unlike the reaction with C60 (Belik et al., Angew. Chem. lnt. Ed. Engl. 1993, 32, 78-80), microwave radiation was required. Similarly, cyclopropanation of malonic acid derivatives could be carried with fullerenes (C. Bingel, Chem. Ber. 1993, 126, 1957-1959.) and ultrashort (20 to 80 nm) carbon nanotubes under mild conditions, whereas microwave-assistance was necessary for longer tubes.